Microfabricated vertical comb actuator using plastic deformation

ABSTRACT

A microfabricated actuator of the vertical comb-drive (AVC) type or staggered vertical comb-drive type for torsional or linear applications includes torsion springs which permit self-aligned deformation of the device (micromirror) structure of the actuator through the heating of the torsional springs to plasticity. The torsional springs can include perpendicular-beam springs or double folded beams which allow axial movement of the spring when heated. Heating of the springs can be by bulk heating of the actuator structure or by Joule heating to the torsional springs by passing an electrical current therethrough.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/851,543, filed May 20, 2004 now U.S. Pat. No. 7,089,666. Theparent application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/476,534, filed Jun. 6, 2003. Both applicationsare incorporated herein by reference for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government funding under Grant (Contract)No. ECS0096098 awarded by the National Science Foundation. TheGovernment has certain rights to this invention.

BACKGROUND OF THE INVENTION

This invention relates generally to microelectromechanical systems(MEMS), and more particularly the invention relates to torsional andlinear vertical comb MEMS actuators and to the microfabrication of theactuator.

Electrostatic comb-drive actuators are used in numerous MEMSapplications where they have demonstrated their capability for extendedranges-of-movement, stable and reliable operation, and designflexibility in different frequency ranges. In particular, many opticalMEMS applications employ comb-drive actuators for torsional and linearmotions. For example, lateral comb-drives and mechanical hinges orlinkages made of polysilicon or single-crystal silicon have beendemonstrated to make torsional actuators. From these applications,linkage- and hinge-designs have been identified as sources forreliability problems and for limitations on maximum frequencies ofoperation. One design for torsional actuators uses vertically alignedcomb-drives to achieve both higher frequencies and larger scan anglesthan those characterizing the planar polysilicon structures. Othervertically aligned comb-drives have employed polysilicon on SOI andsingle-crystal silicon fabricated using wafer bonding.

FIG. 10 is a plan view of an Angular Vertical Comb-drive (AVC) actuatorincluding a device structure 10 (micromirror for example) which issupported by torsion springs 12 on a support layer 14. Typically, layer14 is a silicon on insulator (SOI) layer and device structure 10 andtorsion springs 12 are fabricated therefrom by conventional photoresistmasking and etching techniques. Comb-drive actuators 16 comprisingstationary comb fingers on support layer 14 and interdigitated moveablecomb fingers on device structure 10 rotatably move device structure 10on torsion springs 12 in response to electrical drive signals applied tothe comb fingers on device structure 10 through electrical contacts 18.

The basic fabrication processes for AVCs and SVCs (Staggered VerticalComb-drive) are first to define the stationary and movable combstructures on the same level of silicon layer, and to deflect either thestationary comb structure by the residual stress induced by a metallayer, or else the movable comb structure using the surface-tensionforce that arises as a result of the reflow of a patterned-photoresistlayer. A limitation of the residual-stress method of fabrication is thatthe structure must be sufficiently flexible so that it undergoesappreciable deformation. Designs that meet this requirement aregenerally limited in their ultimate operating frequencies to a fewhundreds Hertz, Hz. The challenging problems for the reflow of polymerhinges have been the control and reliability of the polymer material.

To improve the performance of torsion-bar micro-actuators, AVC-typetorsional actuators that are composed of all-single-crystal-siliconstructures have been fabricated using controlled-plastic-deformation insilicon that is annealed at elevated temperatures. In recent years,polysilicon and single-crystal silicon membranes were demonstrated to beplastically deformed forming hemispherical domed structures as a resultof the pressures of heated gases trapped in a cavity. Plasticallydeformed polysilicon structures have been used in a self-assembled MEMSprocess.

The present invention is directed to a vertical comb type actuator withnovel torsion springs and to a novel method of fabricating the AVC andSVC type actuator including thermal processing of the torsion springs tocause a permanent deflection of the device structure relative to thesupport structure.

SUMMARY OF THE INVENTION

In accordance with the invention a vertical comb-drive actuator (AVC andSVC) has unique torsion springs which when heated allow permanentdeflection of the device structure of the actuator relative to thesupport structure.

In fabricating the actuator, the torsion springs are heated to realizeplasticity in the springs, and then the device structure is deflected onthe torsion springs relative to the device structure. The actuator canbe bulk heated or an electrical current can be passed through thetorsion springs for Joule heating of the springs. A self-aligneddeflection of the device structure is realized by applying a lid capwith extending pillar to the actuator with the pillar deflecting theheated device structure. The heated device structure is cooled and thenthe lid cap is removed with the device structure permanently deflectedon the support structure.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and the appended claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process flow for plastically deforming verticalactuators in accordance with an embodiment of the invention.

FIG. 2 is a graph illustrating stress-strain curve of silicon at anelevated temperature.

FIG. 3 is a photograph illustrating batch processed scanning mirrorsusing plastic deformation of silicon in accordance with the invention.

FIG. 4 is a photograph illustrating thermo-plastically tilted mirror andprecisely aligned vertical combs.

FIG. 5 is a photograph of micro-pillar structures fabricated on a lidwafer.

FIG. 6A, 6B illustrate different forces applied on a device structuredepending on the position and height of the pillar structure.

FIG. 7 is a graph illustrating standard deviation of plastically formedtilting angles for mirrors of difference sizes and shapes.

FIG. 8 is a graph illustrating frequency response of a microactuatorversus scanning angle.

FIG. 9 is a graph illustrating measured scanning angle and resonantfrequency of a microactuator over 5 billion cycles.

FIG. 10 is a plan view of a conventional angular vertical comb-drive(AVC) actuator.

FIG. 11 is a perspective view of AVC actuators and localized heating ina batch for plastic deformation of torsional springs of themicroactuators in accordance with an embodiment of the invention.

FIG. 12 illustrates process flow for batch processing of localizedheating and plastic deformation in accordance with an embodiment of theinvention.

FIG. 13 illustrates two torsional springs for use in an AVC actuator inaccordance with embodiments of the invention for relieving thermalstresses induced by localized heating.

FIG. 14 is a picture of batch processed scanning mirrors using localizedheating of torsional springs in accordance with the invention.

FIG. 15 is pictures of the torsional actuators of FIG. 13 andinterdigitated self-aligned vertical comb fingers.

FIG. 16 further illustrates the two types of torsional actuators.

FIG. 17 illustrates micropillars and electrodes fabricated on a lidwafer in accordance with an embodiment of the invention.

FIG. 18 is a graph illustrating I-V curve for localized Joule heatingand pictures of springs corresponding to each heating zone.

FIG. 19. is a graph illustrating frequency of response of amicroactuator versus scanning angle.

FIG. 20 is a graph illustrating measured scanning angle and resonantfrequency of a microactuator over 6 billion cycles.

FIGS. 21A, 21B are plan views of two different vertically driven linearcomb actuators in accordance with embodiments of the invention.

FIG. 22 is a process flow diagram illustrating use of global heating andlocalized Joule heating in fabricating a linear actuator in accordancewith the invention.

FIGS. 23A, 23B illustrate staggered vertical comb-drive and angularvertical comb-drive, respectively.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The design and fabrication of torsion-bar-supported scanning mirrors inaccordance with the invention are demonstrated as an example ofvertical-comb actuators made by the plastic-deformation process. FIG. 1shows the new process. A torsion-bar-supported plate (which may, forexample, function as a mirror) is formed in a first wafer. Using deepreactive ion etching, or DRIE, projecting pillars are configured in asecond wafer and positioned such that, when the two wafers are stackedtogether, the projecting pillars push on and depress the mirrors on thefirst wafer. The two stacked wafers are annealed at temperatures greaterthan 800° C. causing the torsion bar to be plastically strained so that,after cooling to room temperature and separating the two wafers, themirrors are permanently tilted from their original planar positions.

To start the process, the top- and bottom-combs and mirror plates areinitially defined on the same device layer of an SOI wafer in FIG. 1(a)˜(c). During the DRIE (Deep Reactive Ion Etching) step, not only theactuator itself, but also the depressions are formed. The backsideetching of the substrate wafer underneath the mirror and combs removessufficient silicon to provide clearance for the mirror motion. Thisbackside alignment is not critical and even tens of microns of alignmenterrors are acceptable. A second wafer is processed with protrusions thatalign to depressions in the comb-actuator wafer. A second wafer isconfigured with pillars that displace the comb structures to the desiredtilt angles. The two mated wafers are annealed in a furnace at 800˜900°C. When they are separated, the comb actuators are plastically deformedas shown in FIG. 1( f). Throughout the whole process, no criticalalignment is necessary. Top and bottom combs are self-aligned becausethey are defined in a single masking step, and device and lid wafers arealso self-aligned due to the depressions and protrusions.

In FIG. 2, a typical stress-strain curve of single crystal silicon athigh temperature is shown. At room temperature, silicon is specified asa brittle material with a yield stress of ˜600 MPa. At an elevatedtemperature, the mechanical properties of silicon change dramatically.The maximum yield stress (σ_(m)) decreases due to the increased mobilityof dislocations in the crystal. At a temperature above 600° C., thestructures begin to plastically deform with a much reduced flow stress(σ_(f): the stress needed to continue plastic deformation) instead offracture when the induced stress in the silicon structure exceeds theyielding stress. When the pillar structure on the lid wafer pushes oneedge of the mirror surface, the mirror and moving combs rotate on thetorsional springs inducing stresses greater than σm in the springs byelastic deformation. Stresses are also induced in the mirror plate;however, the magnitude of the stress in the mirror is much smaller thanσ_(m) or σ_(f) according to finite-element analysis and negligiblecompared to that in the torsional springs. Once the temperature isincreased, the stresses in the torsional springs are relieved causingtheir plastic deformation. No deformation of the mirror plate isobserved for the annealing temperature of 800˜900° C.

FIG. 3 is an SEM microphotograph of batch-fabricated micromirrorsproduced by this process. A close-up view of tilted comb fingers ispresented in FIG. 4, showing the precisely aligned vertical comb sets.FIG. 5 shows the lid wafer and pillar structures used to deform themicroactuators in FIG. 3. No damage in the pillars or lid wafer wasobserved indicating that the lid wafer could be used repeatedly.

The maximum initial tilt angle that can be formed by the method ofplastic deformation that we have described above is limited by thefracture strength of the single-crystal silicon. We have found that20˜30° of initial angular displacement is possible with a precise valuedepending on the specific torsion-bar geometry. There are two ways tocontrol the initial tilting angle; (1) by adjusting the height of thepillar structure, and (2) by changing the position on the mirror wherethe pillar structure pushes as can be seen in FIG. 6.

Before high-temperature annealing, when the same height of pillarstructure is used, forcing the mirror structure at position Q in FIG. 6causes a larger tilt than if the pillar were at position P. The tilt inthe second case would, however, be the same if the pillar height weresufficiently increased. Although the tilt angle is the same in these twocases, however, the stresses induced in the torsion bar are not equal.The same angle means the same moment is applied to the torsion bar atposition P and Q, and that isM_(p)=F_(p)x_(p)=M_(Q)=F_(Q)x_(Q)  (1)and considering the stiffness of torsion bar,

$\begin{matrix}{M_{P} = {M_{Q} = {c\frac{G\; J\;\theta}{l}}}} & (2) \\{{therefore},} & \; \\{F_{P} = {{{c\frac{G\; J\;\theta}{l\; x_{P}}} < F_{Q}} = {c\frac{G\; J\;\theta}{l\; x_{Q}}}}} & (3)\end{matrix}$where G is the shear modulus of torsion bar, J is the polarmoment-of-inertia of the torsion bar, l is the length of the torsionbar, and c is a constant depending on the cross-sectional aspect ratioof the torsion bar.

From Eq. (3), it is obvious that pushing the mirror tip with a longerpillar causes smaller vertical force than pushing the position closer tothe torsion bar with a shorter pillar. Because excessive vertical forceon the mirror surface may lead not only to the torsion, but also tounwanted bending of the torsion bar or undesirable curvature of themirror, it is desirable that the pillar structures are designed to beplaced at the tip of the mirrors. The measured vertical deflections oftorsion bars after plastic deformation were less than 0.3 μm, and themeasured radii-of-curvature of the mirrors were in the order of meters,which indicate there can be only very little plastic deformation inducedin the mirror plate.

Considering that actuators are made in a batch, the uniformity of theplastic deformations is another parameter to be characterized. Theplastically deformed angles of seven different designs were measured andthe standard deviations for 10 samples of each type with respect to theaverage values are plotted in FIG. 7. The typical standard deviation isless than 0.1°. Possible sources of deviation are; particles entrappedbetween the device wafer and lid wafer, or the tolerance forself-alignment between the depression on the device wafer and theprotrusion on the lid wafer, which can be easily improved by mating thelid and device wafers with more tight tolerances and in a cleanerenvironment.

The measured resonant frequencies of seven different types of actuatorswith different sizes of torsion bars, mirrors and comb fingers rangefrom 1.90 kHz to 5.33 kHz. The dynamic performance of the type-4actuator with the initial tilt angle of 5.22° is measured and presentedin FIG. 8. Data were collected using 40Vdc and 13Vac drive for themirror actuator with 50 μm thickness of both fixed and moving combs anda mirror size of 800 mm′800 mm. The resonant frequency is measured at2953 Hz, and a maximum scanning angle of 19.2° is achieved. The qualityfactor measured (in air) is 120.

The plastically deformed angular vertical comb actuator is exceedinglyreliable and stable operationally because of its simple and ruggedstructure made of single-crystal silicon. To assess its long-termreliability, the actuator was resonated for more than 5 billion cyclesusing the same driving voltages as are indicated in FIG. 8. The resonantfrequency and scanning angle change were measured periodically (every255 million cycles) and these data are presented in FIG. 9. The maximumvariations of resonant frequency and scanning angle were 0.064% and3.6%, respectively. The error bars on the frequency curve are shownsince the maximum scanning-angle change was undetectable to 0.2 Hzvariation of the operational frequency.

FIG. 11 shows schematically how, in another embodiment of the invention,the torsional springs are permanently deformed in a batch by localizedJoule heating using micromachined silicon pillars and electrodes. Thedevice chip (top) of 1 cm×1 cm and lid chip (bottom) of 1 cm×1.1 cm thatis slightly larger than device chips for wire bonding are stackedtogether using self-aligned structures of key and key-slot pairs. Whenthe pillar structure on the lid wafer pushes one edge of the mirrorsurface on the device chip, the mirror and moving combs rotate inducingstresses greater than am in the torsional springs by elasticdeformation. With proper mechanical flexure designs, the magnitude ofthe stress in the mirror can be much smaller than σ_(m) or σ_(f) andnegligible compared to that in the torsional springs. Electrical poweris then applied from the external power source through the wires andelectrodes on the lid chip, to anchor and torsion beams on the devicechip. Since the torsional springs are designed to have more than 40times larger electrical resistance value than other structures, only thetorsional springs are reaching the temperature high enough to allowplastic deformation of silicon. Using this method, both the requiredstress level and high temperature are induced locally on the springstructures for self-aligned plastic deformation.

The detailed fabrication process is explained in FIG. 12. To start theprocess, the comb actuators and mirror plates are initially defined onthe same highly doped device layer of an SOI wafer in FIG. 12( a)˜(c).During the DRIE (Deep Reactive Ion Etching) step, not only the actuatoritself, but also the keyslots are formed. The backside etching of thesubstrate wafer underneath the mirror and combs removes sufficientsilicon to provide clearance for the mirror motion. This backsidealignment is not critical and even tens of microns of alignment errorsare acceptable. A lid wafer is processed with keys that align tokeyslots on the device wafer; pillars that displace the comb structuresfor desired tilt angles and electrodes that make electrical contactswith anchors on the device wafer to deliver electrical power in FIG.12(1)˜(3). The pillars are designed to be 40 ìm higher than theelectrodes to allow adequate torsional displacements of mirrors duringthe localized plastic deformation process. The lid wafer is then putinto HF to etch buried oxide to form undercut underneath the electrodesstructures, with which, each electrode can be electrically separatedafter the following maskless metallization process. After dicing thedevice wafer into chips and stacking up those on top of lid chips (FIG.12( d)), the following Joule heating process turns elastic strain into aplastic deformation. When they are separated, the comb actuators arepermanently reshaped as shown in FIG. 12( e). Throughout the wholeprocess, no critical alignment is necessary. Top and bottom combs areself-aligned because they are defined in a single masking step, anddevice and lid wafers are also self-aligned due to the keyslots andkeys.

Torsional springs of scanning mirrors are typically designed to be beamswith fixed-fixed boundary conditions with mirrors located at the centerof beams. However, those beams with fixed-fixed boundary conditions maybuckle in an uncontrollable manner in our process due to the thermalexpansion. Our tests with fixed-fixed beams revealed that buckled beamscould not recover their original shape in most cases, causing themisalignment between moving combs and fixed combs.

FIG. 13 shows two improved torsional spring designs in accordance withthe invention to relieve thermal stresses in the process. In type A, thelateral thermal strain is absorbed by perpendicular-beam springs at theend of the torsional spring. The geometrical symmetry of the designassures concurrent heating on both side of the mirror plate to maintainself-alignment during the plastic deformation. The dimension of theperpendicular-beam spring is designed from the buckling criterion oftorsional springs. If the perpendicular-beam spring is too compliant, itmay cause lateral misalignment of the two comb sets. If it is too stiff,it may not work effectively as a thermal stress absorber causing thebuckling of torsional springs. Type B is an alteration of double foldedbeams designed to be flexible in torsion and stiff in lateraldeflections. Each beam has the same dimension to have equivalentelectrical resistances for the same heating and thermal expansioneffects. here the double folded beam includes first and second beammembers coupling the device structure to an apex and third and fourthbeams coupling the apex to the support structures.

FIG. 14 is an SEM of 8 micromirrors produced in a batch by the processdescribed above. Typically, there are 6˜10 micromirrors on a chipdepending on the dimensions of mirror and flexure. The close-up views oftwo different types of actuators are shown in FIG. 15. The lower-leftpicture is the type A design after the localized plastic deformationprocess with the upper-left picture showing close-up view of theoffsetting vertical comb fingers and the upper-middle picture showingthe close-up view of the, perpendicular-beam spring. The type B designis shown in the lower-right picture with the upper-right picture showinga close-up view of the double folded torsional spring.

A white light interferometric measurement after the plastic deformationis shown in FIG. 16, for two different designs. The measured radii ofcurvature of type A and B mirrors after localized plastic deformationwere larger than 1.4 m and 2.0 m, respectively, representing excellentmirror flatness after the process. FIG. 17 is the side view SEM pictureof the lid chip with pillars and electrodes used to deform the torsionalsprings. The top surface of pillars and keys are on the same level andthe electrodes are 40 μm below them. No damage on the lid chips wasobserved indicating that the lid chips could be used repeatedly.

FIG. 18 shows the appropriate range of power to induce plasticdeformation on type B actuators by localized heating. Each of the threecurves on the graph represents I-V measurements when one, two and threedevices of the same layout are heated in order counted from the bottomcurve. As shown in the plot, the amount of current flow for two andthree devices processed at once is linearly proportional to the currentof a single device, when the power is driven by constant voltage mode.This is due to the parallel connection of each device to the electrodessuch that the current flowing on each device is same under the samedriving voltage.

The I-V curves show linear characteristics under low power input. As thetemperature increases from resistive heating, the resistivity of theheated silicon, which is a function of mobility and number of carriers,increases due to the decreased mobility showing the non-linear behaviorin the IV plot. However the resistivity decreases when the temperatureis very high, as the effect of increased number of carriers becomesdominant. Experimental tests on single crystal silicon show that bestresults (picture A and corresponding voltage range A in FIG. 18) areachieved at the power range of 500˜600 mW/device, while higher powercauses migration of silicon molecules resulting in degradation andnon-uniform spring shapes (picture B and corresponding voltage range B)and eventually melting of structures (picture C and correspondingvoltage range C). In all the aforementioned experiments, only a fewseconds of power input was enough for plastic deformation. The dynamicperformance of the actuators are measured and presented in FIG. 19. Datawere collected using 30V_(dc) and 15Vpp drive on a type A actuator with600 μm×720 μm×50 μm size mirror and 680 μm×10 μm×50 μm size torsionalsprings. The number of comb pairs of the actuator is 100 and the gapbetween fixed comb and moving comb is 6 μm. The resonant frequency ismeasured at 4132.6 Hz, and a maximum optical scanning angle of 47.3° isachieved. The quality factor measured is 202 in air.

In FIG. 20, the reliability test results are presented for a type Aactuator with the same dimension as the one tested in FIG. 19, but fromdifferent batch. The actuator was operated at its resonance for morethan 5 billion cycles at the operation voltage of 30Vdc+14Vpp, and theresonant frequency and scanning angle were measured at every 178 millioncycles which corresponds to 12 hours of operation. As shown, themeasured initial scanning angle was 50.9° at the frequency of 4136.1 Hz.Overall, both frequency and scanning angle decreases slightly as theoperation cycle increases. However, the variations of frequency andangle during the 6 billion cycles of continuous operation are smallerthan 0.073% and 0.4% of their initial values, respectively. From theexperiment, it was observed that both frequency and amplitude tend tochange concurrently in the same incremental/decremental direction. It isnot obvious if this variation is from the degradation of plasticallydeformed silicon material or from other possible effects such ashumidity and temperature change or particle contamination, since theexperiment was performed with a mirror actuator unpackaged and in thelab. However, considering the extremely small amount of variation, thescanning mirror actuators made by localized plastic deformation arerobust and reliable.

The previous examples of scanning micromirrors and variable capacitorsare torsional actuators where torsion bars are plastically twisted toestablish vertically interdigitated comb finger sets. If one changes themechanical flexures to different designs, linear actuators moving in thez-direction can be made using the self-aligned plastic deformationprocess. In this case, the springs should be designed to have bendingstresses larger than the yield stress at an elevated temperature. Unlikethe angular vertical comb shape in torsional actuator, this will formstaggered vertical comb (SVC) sets. FIGS. 21A, 21B show the top views oftwo design examples for vertically driven linear comb actuator. Bothdesigns are for linear out-of-plane motion with interdigitated combfingers and have either silicon Fresnel lenses or polymer lenses at thecenter, which is useful for a 3-D optical scanner or micro-opticalsystems that need vertical focal point adjustment of lenses. For stablelinear motion, the mechanical flexures must be flexible only in thez-direction and stiff enough in other directions. The spring in FIG. 21Ais a double folded beam modified from the typical surface micromachinedcomb-drive design—moving structures encloses the anchors. This designhas all anchors on the perimeter of the comb structure to have a firmconnection to the solid boundary but allows the comb-structure to movevertically when the backside hole is etched. Since one end of the doublefolded beam is not connected to the fixed boundary, it is free ofthermal stresses for localized heating to relieve thermal stress.Therefore, only double folded beams are heated and plastically deformedfor polymer lenses to be mounted by coating and lithographicallypatterning photosensitive polymers even before the plastic deformationprocess. This batch process of lens forming is possible only in thelocalized heating case. Patterned lenses may form perfect dome shape byreflowing them at temperatures lower than 200° C. The spring design inFIG. 21B is for global heating as described earlier. Four sets ofsprings are supporting the movable structure and each set is composed oftwo torsion bars and a rigid connection beam. The rotational motion ofthe torsion bars is transformed to translational motion through therigid beam. The spring stiffness in z-direction can be easily adjustedby changing the length of rigid beam. The torsion bars are notcompletely straight and designed to be initially bent at the middlepoint, which increases in-plane stiffness.

FIG. 22 shows the fabrication process for a vertically driven linearactuator. It is similar to the plastic deformation process described forthe torsional actuator, but the pillar structure deforms the linearspring in a z-direction (vertical direction). Multiple pillars on thelid wafer can be used to guarantee a flat surface of the structure wheremicrolenses are located. After plastic deformation, the moveablestructure generates linear translational motion in the out-of-substratedirection.

FIG. 23A illustrates a staggered vertical comb-drive, and FIG. 23Billustrates an angular vertical comb-drive. The linear actuatordescribed here adopts SVC, while the torsional actuator described abovecan be actuated by either AVC or SVC. Both AVC and SVC can be formed bythe plastic deformation processes with global heating or localizedheating in accordance with the invention.

Plastic deformation of silicon by batch heating and by localized Jouleheating was successfully applied to build micro scanning mirrors in abatch process that does not demand on any critical alignment steps. Thenovel designs for torsional springs prevent the buckling problem fromnon-uniform thermal expansion by localized heating. The optimal powerrange of localized heating was characterized and the measured dynamicperformance of the actuator was recorded.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art with departing fromthe true spirit and scope of the invention as defined by the appendedclaims.

1. A vertical comb-drive actuator comprising: a) a support structure, b)a planar device structure, c) a plurality of torsion springs physicallycoupling the device structure to the support structure, the torsionsprings being movable on deflecting the device structure in response toelectrical actuation, each torsion spring having an end portion forjoining the support structure which permits axial movement of thetorsion spring in response to thermal expansion of the torsion springduring fabrication, said end portion further comprising double foldedbeams for coupling the device structure to the support structure, thedouble folded beams including first and second beam members coupling thedevice structure to an apex and third and fourth beams coupling the apexto the support structure.
 2. The actuator as defined by claim 1 whereinthe device structure includes a first plurality of combs and the supportstructure includes a second plurality of combs which are interdigitatedwith the first plurality of combs and which cooperatively drive thedevice structure in response to electrical actuation.
 3. The actuator asdefined by claim 1 wherein the actuator comprises semiconductormaterial.
 4. The actuator as defined by claim 3 wherein the devicestructure and the torsion springs comprise doped silicon.
 5. Theactuator as defined by claim 4 wherein the actuator comprises a siliconon insulator substrate.
 6. The actuator as defined by claim 4 whereinthe device structure includes a first plurality of combs and the supportstructure includes a second plurality of combs which are interdigitatedwith the first plurality of combs and which cooperatively drive thedevice structure in response to electrical actuation.
 7. The actuator asdefined by claim 4 wherein the actuator comprises an angular verticalcomb-drive actuator.
 8. The actuator as defined by claim 4 wherein theactuator comprises a staggered vertical comb-drive actuator.
 9. Theactuator as defined by claim 4 wherein the actuator comprises atorsional actuator.
 10. The actuator as defined by claim 4 wherein theactuator comprises a linear actuator.